1. Field of the Invention
The present invention relates to an induction motor, more particularly, to a method of operating a slip angular velocity of an induction motor, on the basis of the voltage, current and velocity of the induction motor which compensates the slip angular velocity in case of vector control of the induction motor by a current controllable inverter.
2. Description of the Prior Art
In general, an induction motor is considered a typical brushless ac motor of which any one of a rotator and a stator is connected to a power voltage while the other is operated by way of an induced voltage.
For example, a velocity control device of such an induction motor is disclosed in Japanese Patent laid-open publication No. sho 57- 180387, which is shown in FIG. 1. More specifically, the device has a variable voltage and variable frequency control portion VVVF provided at the side of a power voltage of an induction motor IM, a velocity operation portion SOP for calculating a difference between a velocity signal NF of the rotational velocity of the induction motor IM and a velocity command signal NR, A current operation portion IOP obtains a current value of the induction motor IM on the basis of an effective current signal I2. An excitation current IO setting signal and the like outputted from the velocity operation portion SOP controls an output voltage of the variable voltage and variable frequency control portion VVVF. Also included is an adder ADD which adds the velocity signal NF and the effective current I2 outputted from the velocity operation portion SOP to control an output frequency of a voltage to frequency converter VFC which applies its output frequency to a frequency control portion FCT. A comparator CP feeds back a primary current IF signal to the velocity operation portion SOP in accordance with an output signal of a polarity detecting circuit PD when the primary current IF detected by a current transformer CT disposed at an output end of the variable voltage and variable frequency control portion VVVF and passed through a rectifier REC is above a predetermined set value. A differentical circuit DF differentiates the output of the adder ADD and feeds the differentiated value to the velocity operation portion SOP. An operation portion OP receives the primary current signal passed through the rectifier REC and the output signal of the current operation portion IOP and supplies the resultant value to a voltage control portion VCT which outputs a voltage control signal to the variable voltage and variable frequency control portion VVVF.
With the velocity control device of the induction motor described above, the motor is rotated with a predetermined slip according to the frequency of the power voltage. Particularly, when the velocity varies, for example when acceleration or deceleration occurs it is difficult to maintain the slip of the motor constant. Thus the rotational velocity of the motor is unstable. A vector control method needs to be adapted to overcome the variable slip and unstable rotational velocity of the motor which occurs due to the variation of the velocity. The method is needed to execute a slip angular velocity operation, so a slip value is obtained when the parameter, such as the resistance of the rotation variable according to the environs temperature is likely incorrect. The method overcomes the problem that normal efficiency of the induction motor cannot be assured.